1. Field of the Invention
The present invention relates to valve assemblies that control the flow of fluid to a hydraulic load, such as a cylinder and piston combination; and more particularly to such assemblies that incorporate electrohydraulic valves.
2. Description of the Related Art
A wide variety of machines have working members that are driven by hydraulic motors, such as cylinder and piston assemblies. Each cylinder is divided into two internal chambers by the piston and selective application of hydraulic fluid under pressure to either of the chambers moves the piston in a corresponding direction. While that action is occurring, fluid is being drained, or exhausted, from the other cylinder chamber to a tank for the hydraulic system.
Traditionally the flow of hydraulic fluid to and from the cylinder was controlled by a manually operated valve, such as the one described in U.S. Pat. No. 5,579,642. There is a trend away from manually operated hydraulic valves toward electrohydraulic valves which are electrically controlled. This change in technology facilitates computerized regulation of various machine functions. Electrical control also simplifies the plumbing of the hydraulic system, as the control valves can be located near each cylinder and not at the operator station. Thus only a single pair of pump and tank lines needs to be run to the hydraulic actuators throughout the machine. Although separate electrical wires may have to be run to each valve, those wires are easier to run and maintain as compared to hydraulic lines.
U.S. Pat. No. 6,073,652 describes an electrohydraulic valve assembly which utilizes four solenoid operated proportional control valves. One pair of valves controls the flow of fluid to and from one of the cylinder chambers, while the other pair of valves controls the flow of fluid to and from the other cylinder chamber. In each pair, one valve regulates the flow of hydraulic fluid from the pump supply line to the associated cylinder chamber and the other valve of the pair controls the flow of hydraulic fluid from that chamber to the system tank. Therefore the cylinder is operated by activating one valve in each pair to apply pressurized fluid to one chamber of the cylinder and drain the fluid from the other chamber. The particular combination of electrohydraulic valves that are activated determines the direction in which the piston is driven.
One drawback of this type of assembly is that four electrohydraulic proportional valves are required to produce bidirectional movement of the piston.
The present invention provides a control valve assembly that utilizes three electrohydraulic valves to control the flow of fluid between a hydraulic motor and both a source and a tank.
That valve assembly includes a fluid supply line for receiving pressurized hydraulic fluid from the source and a return line for connection to the tank. A first intermediate conduit and a second intermediate conduit also are provided.
The valve assembly has first and second work ports for connection to the hydraulic motor, which may be a cylinder and piston assembly for example.
A direction control valve is connected to the fluid supply line, the return line and the first and second intermediate conduits, and is selectively operated between first and second positions by an electrical control signal.
The first and second positions provide different fluid paths between the supply and return lines and the first and second intermediate conduits. In one preferred embodiment, the fluid supply line is coupled to the first intermediate conduit and the return line coupled to the second intermediate conduit when the direction control valve is in the first position, and the fluid supply line is coupled to the second intermediate conduit and the return line coupled to the first intermediate conduit when the direction control valve is in the second position. Another embodiment of the direction control valve has a position in which regeneration occurs where fluid draining from the motor into one work port is supplied to the other work port.
A bidirectional first control proportional valve is connected between the first intermediate conduit and the first work port to control a flow of hydraulic fluid there between. A bidirectional second proportional control valve control the flow of hydraulic fluid between the second intermediate conduit and the second work port.
The direction control valve is operated to determine into which work port pressurized fluid from the source is supplied and from which work port fluid is exhausted. This determines the direction in which the motor operates. The first and second proportional control valves operate to meter the flow of hydraulic fluid to and from the work ports and thus control the rate of movement of the motor. Therefore, the present assembly of three valves achieves the same degree of control over the operation of the motor as prior assemblies having four proportional valves.